1. Field of the Invention
The present invention relates to a purification of crude .beta.-naphthol obtained via an alkylnaphthalene hydroperoxide, and more particularly it relates to a method for efficient removal of by-products, including .alpha.-naphthol, which are difficult to separate by conventional methods from the crude .beta.-naphthol.
2. Description of the Prior Art
At present, the most common method for preparing .beta.-naphthol is a sulfonation of naphthalene followed by fusion. However, many problems are now encountered with this method in terms of manufacturing cost, mass productivity and pollution control measures. Recently, a new industrially advantageous synthetic method for preparing .beta.-naphthol via an alkylnaphthalene hydroperoxide is attracting attention in this field. The alkyl group referred to herein for the alkylnaphthalene hydroperoxide includes an isopropyl, sec-butyl and cyclohexyl group, of which the isopropyl group is most common. Therefore, hereinafter the description will be given in terms of isopropylnaphthalene hydroperoxide.
This method of synthesis of .beta.-naphthol via an isopropylnaphthalene hydroperoxide comprises three reaction steps.
(1) The first step is a preparation of isopropylnaphthalene from two starting materials, naphthalene and propylene, in the same manner as in the preparation of cumene from benzene and propylene. For example, the reaction can be conducted using aluminum chloride as a catalyst at 50.degree. C. to 150.degree. C.
(2) 2. The second step is an oxidation of the isopropylnaphthalene thus obtained to isopropylnaphthalene hydroperoxide (referred to as N-HPO hereinafter) with oxygen or an oxygen-containing gas at 70.degree. to 120.degree. C. in the presence of an alkali.
(3) The last step is a cleavage of the N-HPO thus obtained into .beta.-naphthol and acetone by treating the N-HPO at 50.degree. to 120.degree. C. in the presence of an acid catalyst such as sulfuric acid.
In this process, the reaction product after the cleavage contains acetone, .alpha.-naphthol, isopropylnaphthalene, isopropenylnaphthalene, acetylnaphthalene, dimethylnaphthyl carbinol (which is easily dehydrated to isopropenylnaphthalene at a relatively high temperature) and a tarry material in addition to .beta.-naphthol. Of these by-products, acetone can easily be removed by distillation since acetone has a boiling point greatly different from those of the other by-products. Isopropylnaphthalene and isopropenylnaphthalene can also be removed relatively easily by rectification or other operations. However both .alpha.-naphthol and acetylnaphthalene have a boiling point very close to that of .beta.-naphthol and moreover have a very high boiling and melting point. More specifically, the boiling points of .alpha.-naphthol and .beta.-naphthol are 288.degree. C./760 mmHg and 295.degree. C./760 mmHg, respectively, and the melting points are 96.degree. C. and 123.degree. C., respectively. Thus, it has been considered to be very difficult to separate .beta.-naphthol alone in a high purity by rectification from a mixture with these by-products. In other words, the preparation of .beta.-naphthol via the hydroperoxide route has been considered to be very impractical as an industrial process, unless an advantageous purification of .beta.-naphthol from the cleavage product could be established.
In this process via the hydroperoxide, it is unavoidable for .beta.-isopropylnaphthalene to be contaminated with a certain amount of .alpha.-isopropylnaphthalene produced as a by-product. Therefore, the resulting hydroperoxide is also a mixture of .alpha.- and .beta.-isomers, resulting in the formation of .alpha.-naphthol as a by-product in the last reaction step.
Consequently, for the purpose of producing .beta.-naphthol industrially advantageously, it is desirable to control the content of the .alpha.-isopropylnaphthalene in the .beta.-isomer used as a material for oxidation to as low a level as is possible. Previously, a process was found in which the production of .alpha.-isopropylnaphthalene as a by-product could be controlled to a small extent, but preparation of the .beta.-isomer alone is impossible industrially and contamination of the .beta.-isomer with a certain amount of the .alpha.-isomer can hardly be avoided. Therefore, .alpha.-N-HPO is also produced by oxidation and thus the resulting naphthol from the cleavage is a mixture of .alpha.- and .beta.-isomers. This means that purification of .beta.-naphthol from the cleavage product requires removal of .alpha.-naphthol from the .beta.-isomer thereof.
Separation of .beta.-naphthol by a countercurrent extraction process is disclosed in U.S. Pat. No. 2,727,927, but removal of .alpha.-naphthol which is a serious problem is very difficult by this process.
Furthermore, U.S. Pat. No. 3,076,035 discloses a method for separating .beta.-naphthol from a mixture of the .alpha.-isomer and the .beta.-isomer by subjecting the mixture to a recrystallization treatment using an aqueous sodium hydroxide solution. However, this method is not advantageous from the standpoint of cost since a large amount of sodium hydroxide is used and moreover the after-treatment is so troublesome that the method can not be employed industrially.